Obesity is sometimes called one of society’s “last acceptable prejudices.” Fat people are often treated horribly, even with the body positivity movement surfacing in our Instagram feeds. In the U.S., overweight people are often perceived as weak, lazy selfish and lacking willpower.

And like most prejudices, science suggests that impression is dead wrong.

In growing numbers, studies are identifying possible links to obesity that go way beyond food and exercise: Chemicals found in common plastics, hormones released from chronic stress and genetic factors only scratch the surface of potential factors. It’s too soon to say whether all these links are real, but scientists’ curiosity certainly raises these questions: What do we really know about being fat, and why do we fat shame?

These days, even our most conventional understanding of weight loss — “calories in, calories out” — is being challenged by obesity researchers.

“I think our field is in a bit of a schizophrenic mindset. On one hand, we have known for nearly a century that body weight is controlled through a complex system of hormones and neurological influences,” David Ludwig, Harvard University professor, Boston Children’s Hospital physician and author of Always Hungry?, said. “But at the same time, we’ve resorted all too often to the same simple admonition. ‘Eat less, move more. All calories are alike, just shift your calorie balance and you’ll take care of the problem.’ These two perspectives don’t co-exist.”

America’s big bigots

Fat prejudice is a paradox in the United States because, simply put, most people are technically fat. Nearly 71% of all adults in the U.S. over age 20 are overweight or obese, according to the Centers for Disease Control and Prevention.

Yet somehow, overweight people are still singled out and branded as unlikeable and morally inferior. A famous experiment from 1961 is a haunting example of how these decade-old prejudices affect us from our youngest days: Researchers asked six groups of children from various backgrounds to rank the likability of other hypothetical children depicted in drawings. All of the groups consistently ranked a drawing of a fat child as least likable, behind illustrations of a child in a wheelchair, one with crutches and another with a missing hand.

Of course, that was the 1960s. Americans have only gotten fatter since then, but it looks like prejudices haven’t budged much. In a 2016 study from the U.K., researchers read a book about characters named Alfie or Alfina to more than 250 kids (ages 4 to 6) in two separate studies. Versions varied between groups, but Alfie or Alfina were either normal weight, overweight or in a wheelchair; Alfina sometimes had fat friends, too. “When forced to choose between characters, fat ‘Alfie’/’Alfina’ was rejected on most constructs,” researchers wrote.

By their teenage years, most Americans will see an overweight person get bullied. Roughly 88% of the 1,555 high-school students surveyed in a 2010 study said they had witnessed overweight peers be “teased in a mean way” or “teased during physical activities,” while 54% reported witnessing physical harassment.

In the adult world, things don’t appear to be much better. Studies have repeatedly shown weight discrimination is an issue for job-seeking Americans, particularly women. The issue is also highly intersectional: Communities with a high number of black residents tend to have lower-quality retail food environments (i.e. fewer grocery stores nearby), while Hispanic and black populations in the U.S. tend to have higher rates of obesity. Low-income towns tend to have “less open space, a greater density of fast-food restaurants and higher rates of overweight [people or] obesity,” a 2009 study concluded.

But would prejudices change if we knew obesity wasn’t necessarily a person’s fault?

“The conventional ‘calorie in, calorie out’ perspective can’t explain the obesity epidemic. They have to write it off to somehow people are [lazy],” Ludwig said. “It just isn’t on our willpower. It’s our biology … After enduring rising hunger and a slowing metabolic rate, you feel miserable and eventually your metabolism wins.”

America’s serious gains

In 1960, the average American woman weighed 140 pounds — as of 2015, that number had risen to around 166.2. In the same time period, the average man went up from 166.3 pounds to 195.5. That’s a roughly 18% increase in body weight for both genders, and researchers don’t always agree on why it happened.

“The fact that there are all these ideas out there and all these potential factors ... we might not really know what’s going on yet,” Dr. Mark Friedman, vice president of research at the Nutrition Science Initiative in San Diego, said by phone. “There’s also a lot of problems with the quality of studies that have been done on what diet to eat.”

Former first lady Michelle Obama gets ready to dance during a Lets Move! event at a school on Feb. 9, 2012, in Des Moines, Iowa.Carolyn Kaster/AP

The low-fat diet problem

The average American has had a weight problem for decades, creating a $64 billion weight loss industry that still — perplexingly — hasn’t made a significant, measurable difference on our waistlines. Part of the reason is that Americans were given bad advice on what to eat, Ludwig said, and he’s taking aim at the popular low-fat diet.

“We have plenty of evidence that the low-fat diet that was recommended for 40 years failed,” he said. “That needs to be understood. The persisting damage of the low-fat diet needs to be reversed.”

The basic argument is this: When low-fat foods hit the market en masse (Snackwells, anyone?), food manufacturers merely replace the fat with sugar. That’s a problem because sugars and refined carbohydrates — also sugars — cause a spike in insulin, a hormone that turns carbs into fat, promotes the storage of fat and inhibits fat-burning.

“You’re not only causing more fat to be made, but you’re blocking the utilization of fat in your body — the oxidation of burning fat in your body,” Ludwig said. “One needs to look no further than a child with type 1 diabetes. It’s a condition where the pancreas can’t make enough insulin … These kids can be eating thousands of calories a day and they’re still losing weight. Put the child on the right amount of insulin and weight trajectory returns to normal.”

A growing number of scientists are trying to shift dieters’ focus away from calorie-counting and processed carbohydrates.Tim Roske/AP

Meanwhile, foods high in fat — avocados, nuts, olive oil — give people energy but don’t necessarily promote fat storage in the same way processed carbs and sugars do.

“Those [fat-based] calories stick around the bloodstream longer, support the metabolism and reduce hunger,” Ludwig said. “We need to embrace this. We need to admit our error, that the advice for 40 years was wrong, and we need to reverse that. It’s not something that medical societies or the government likes to do.”

Obesity might cause overeating — not the other way around

What Ludwig believes about weight gain is contrary to what we’re often told: We’re not fat because we overeat; we overeat because we’re fat. He uses pregnancy as an analogy — as the fetus grows, the mother eats more.

“In the same way, if our fat cells have been overstimulated, if they’re growing too fast and taking in too many calories of the food that you’re eating, there are actually too few calories for the rest of the body and that’s why we get hungry and eat more,” he said. “If we resist that desire [with low-calorie diets], then our body fights back in other ways, such as slowing down the metabolism.”

So, if the process of becoming obese makes us eat too much, then why are we becoming obese in the first place?

The answer, according to Ludwig and Friedman, is fat accumulation caused by diets high in sugars and processed carbs — “especially when coupled with high levels of fat,” Friedman said. “The food we’re eating is what’s driving us to eat more. That’s the case for many people. There also could be an interaction with your physiological ... some people are going to be more prone to obesity than others.”

Other possible causes for weight gain

There are other reasons for being fat than just food and exercise alone. Some people gain weight because they have an untreated medical condition.

Hypothyroidism — which affects nearly 5% of Americans 12 and older — slows down the metabolism. That can make weight easier to gain and harder to lose, regardless of what the individual eats. Meanwhile, pituitary tumors, Cushing syndrome and polycystic ovary syndrome can also spur weight gain, among many other diseases. Meanwhile, some people may put on weight because of medication they’re taking for blood pressure, depression, epilepsy, diabetes or mental disorders like schizophrenia.

In 2012, scientists reviewed 240 research papers and found that exposure to certain chemicals throughout life — including in the womb — could disrupt the body’s insulin function and cause obesity in animals (this process affects fat storage, among other things). Many of the chemicals studied come from pesticides, diesel exhaust, antipsychotic drugs and plastics. They included lead, bisphenol A (known as “BPA” and commonly found in plastic), brominated flame retardants (used in plastics, textiles and electronics), polychlorinated biphenyls (found in oil paints, plastics, floor finish, old electronics, motor oil and cable insulation).

“My guess is that if there was a really big effect there, we’d all know about it and there wouldn’t be controversy over it,” Friedman said.

The jury is still out on much of this work, and continuous research is needed to confirm connections. Regardless, it’s clear weight gain and obesity are still explorable territory for scientists — and that a person’s weight could be a more complicated story than how many calories they eat and how much they move.